1. Field of the Invention
This invention generally relates to systems and methods for inspecting a wafer. Certain embodiments relate to a wafer inspection system that can include various optical elements and polarizing elements, which in combination segment the collection numerical aperture of a collection subsystem thereby optimizing the system for detection of certain defects while also possibly suppressing detection of other defects.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Examples of commercially available wafer inspection systems include the Surfscan SP1, SP2, and SP3 systems, which are commercially available from KLA-Tencor, Milpitas, Calif., and which generally are single-spot, spiral-scanning systems using an ellipsoidal collector and a supplementary small lens collector that fills in the central numerical aperture (NA) portion missing from the ellipsoidal collector. Various examples of such systems are illustrated in commonly owned U.S. Pat. No. 6,201,601 to Vaez-Iravani et al., which is incorporated by reference as if fully set forth herein.
Some inspection systems are designed such that different detectors detect light scattered into separate, different parts of the collection NA of the system. For example, inspection systems that use acousto-optical device (AOD) spot scanning with multiple (e.g., 5) lens collectors dividing up the collection NA are described in commonly owned U.S. Pat. No. 7,605,913 to Bills et al., which is incorporated by reference as if fully set forth herein. Additional examples of inspection systems that use spiral spot scanning with multiple detectors (e.g., 8) dividing up the collection NA are shown and described in U.S. Pat. No. 7,616,299 to Okawa et al., which is incorporated by reference as if fully set forth herein. There are similar systems that use multiple lens collectors to divide up the full collection NA based on the same concept. U.S. Patent Application Publication No. 2009/0284737 to Matsui, which is incorporated by reference as if fully set forth herein, describes a concept of segmented collection NA by splitting a mirror collector. In addition, commonly owned U.S. Pat. No. 6,538,730 to Vaez-Iravani et al., which is incorporated by reference as if fully set forth herein, describes a way of achieving collection NA segmentation using fiber arrays.
Some inspection systems are designed to use one or more polarizers to suppress surface scattering from a wafer, possibly in combination with segmenting the collection NA of the system. For example, U.S. Pat. No. 6,034,776 to Germer et al., which is incorporated by reference as if fully set forth herein (hereinafter “Germer”), discloses the use of a polarizer to null the scattering from surface roughness. Three collection configurations are described, one embodiment that uses multiple collectors, another embodiment that uses fibers, and a third embodiment that uses a mirror collector. Commonly owned U.S. Pat. No. 7,436,505 to Belyaev et al., which is incorporated by reference as if fully set forth herein, discloses a computer-implemented method for maximizing signal-to-noise by configuring portions of the scattering hemisphere using an ellipsoidal mirror collector. Commonly owned U.S. patent application Ser. No. 12/618,620 by Biellak et al. filed Nov. 13, 2009, issued as U.S. Pat. No. 8,169,613 on May 1, 2012, which is incorporated by reference as if fully set forth herein, discloses a segmented polarizer mask with arbitrary polarization.
Additional inspection systems are designed to reduce surface scattering by reducing the size of the illumination spot on the wafer and compensating for the reduced size of the spot by illuminating multiple spots on the wafer simultaneously. For example, commonly owned U.S. Pat. No. 7,358,688 to Kadkly et al., which is incorporated by reference as if fully set forth herein, discloses oblique one-dimensional multiple spot arrays with a lens collector. The systems include a unique illumination lens design that generates the one-dimensional spot array for oblique illumination. The illumination optics include tilted/decentered aspheric elements to generate the one-dimensional spot array that is tilted with respect to the tangential direction so that each spot scans adjacent tracks, while the incident plane is parallel to the radial direction of the wafer. Also shown in the patent is a high NA lens collector concept. Commonly owned U.S. Pat. No. 7,489,393 to Biellak et al., which is incorporated by reference as if fully set forth herein, discloses another way to generate a one-dimensional spot array at an oblique illumination angle. The proper tilt angle of the spot array with respect to the tangential direction is achieved by tilting the incident plane with respect to the radial direction.
Some of the systems described above have a number of disadvantages. For example, some of the systems described above are optimized (e.g., for maximum signal-to-noise ratio (SN), minimum haze, or maximum capture rate) based on mapping of surface scattering on the scattering hemisphere in spherical coordinates. For example, Germer discovered that the polarization of surface scattering changes with scattering angle and therefore proposed using multiple collectors distributed over the scattering hemisphere so that each one can be optimized independently to accommodate the change of polarization from surface scattering. In particular, Germer states:                It is beneficial to employ as many individual collection systems as possible, thus reducing the solid angle ‘seen’ by each; by doing so, the total system will better discriminate against surface microroughness, since the polarization due to microroughness will vary over any finite solid angle. For a finite solid angle, the discrimination is limited by the changing polarization state over that solid angle. (Germer—col. 7, lines 17-23).Some inspection system architectures described in the above-referenced patents using multiple collectors to divide up the scattering hemisphere seem to be heavily influenced by Germer's arguments.        
The disadvantages of the above-referenced systems can be described based on the similarities among the various inspection systems. For example, for “hard-wired” segmentation of the collection NA (e.g., collection that uses multiple lens collectors to divide the hemisphere of scattering into multiple segments), one disadvantage that such systems have in common is that the segmentation of the NA is fixed and is therefore difficult to reconfigure to optimize the systems for different samples and defects. In this manner, such configurations may have less than optimum performance for various defect types. Another disadvantage of such systems is that the majority of the collector optics is at a tilted angle with respect to the wafer surface normal. Therefore, it is difficult, if not impossible, to image multiple spots onto a detector array. As such, these configurations are not compatible with multi-spot illumination. In addition, the collection is also less efficient due to gaps between collectors.
For mirror-based, large NA collection (e.g., collection that uses a single large NA collector that is based on a mirror, e.g., ellipsoidal or parabolic), the disadvantage of using a mirror collector is that the polarization changes upon reflection due to the substantially large phase shift between p and s polarization. This effect scrambles the well-aligned nearly linear polarization of surface scattering from smooth silicon wafer surfaces, making it substantially difficult to use a polarizer to suppress the surface scattering and therefore undermines a capability to improve defect sensitivity.
With regard to using polarized collection to null surface scattering, Germer first disclosed the method of using polarization of collection to suppress the surface scattering. Some of the other patents referenced above also disclose the use of orthogonal polarization of detection to improve signal-to-noise for specific defects. There are two disadvantages to this approach: 1) the collection hemisphere is divided into multiple separate collection solid angles rendering the system complex and the collection efficiency low; and 2) the polarization of scattered light is mapped onto the surface of a hemisphere, across which the polarization of scattered light changes with scattering angle. This second disadvantage is the main reason that in previously used systems the hemisphere has to be divided up (so that over each relatively small aperture, the polarization of surface scattering is approximately aligned).
With regard to the previously used multi-spot illumination systems, in the existing one-dimensional multi-spot illumination concepts, each spot scans adjacent tracks on the wafer, which requires magnification (in addition to beam size expansion) changes for both illumination and collection when the spot size changes. For example, the spacing in the radial direction between spots needs to change when the spot size changes, which can result in pitch changes between tracks. These and other disadvantages of currently used multi-spot inspection systems are described in commonly owned U.S. Patent Application Publication No. 2009/0225399 to Zhao et al., which is incorporated by reference as if fully set forth herein.
Accordingly, it would be advantageous to develop an architectural approach for future generations of bare wafer inspection systems that enables significant improvements in the achievable signal-to-noise ratio for substantially small particles on bare wafers without one or more of the disadvantages of the currently used inspection systems.